An inherent characteristic of large bore direct injection diesel engines is the very short time available for the satisfactory introduction of fuel into the combustion chamber. This often leads to the phenomenon of ignition delay which causes physical damage to the diesel engine as it forces peak cylinder pressure far higher than ordinarily desired. Since the inception of diesel engines, substantial efforts have been placed in reducing the detrimental effects of ignition delay.
Fuel technologists have attempted in part to decrease the phenomenon of ignition delay by raising the cetane value of diesel fuels. Various engineering techniques have also been employed to reduce this phenomenon. While some progress has been made, ignition delay still persists.
Ignition delay is a chemical phenomenon caused by the fact that before any liquid hydrocarbon fuel can spontaneously ignite from compression heat, it must first vaporise and gather sufficient heat from the air to raise it well above its self-ignition temperature.
While in theory this may seem straight forward, it is compromised by the very short time available during the conventional diesel cycle within which to vaporise the fuel. Typically, the fuel injection period for a diesel engine is about 15 crank degrees before top dead center (TDC). Thus even for a low speed diesel engine operating at, say 1500 rpm, this time period is less than two thousandths of a second.
The situation is further aggravated by the fact that the liquid fuel when it evaporates can absorb a disproportionate amount of compression heat, and due to its much higher mass than the air, there is a reduction in the rate of heat transfer from the air to the fuel.
As a consequence, instead of the injected fuel burning as soon as it is introduced into a main combustion chamber of the diesel engine, and thus liberate the heat value of the fuel at a controlled combustion rate, the ignition delay allows the fuel to accumulate such that upon finally reaching self-ignition temperature, all the accumulated fuel present along with that being injected tends to explode in an uncontrolled manner rather than steadily burn. Consequently, very high peak cylinder pressure is created just prior to TDC which also coincides with compression pressure reaching its maximum. When close to TDC, due to the alignment of crank bearings, fast and sympathetic expansion of the combustion chamber is not possible. Thus, in maintaining pressure and temperature equilibrium, a substantial proportion of the heat generated by the fuel is simply transferred to other engine components rather than being directed to useful work.
Further, the high peak pressure at TDC heavily loads the piston rings of the engine against its cylinder walls at a time when they are momentarily stationary (that is, changing from the upward compression stroke to the downward expansion stroke) and therefore unable to generate a hydrodynamic lubrication film. This leads to unprotected metal-to-metal attrition further contributing to engine wear.
The present invention was developed with a view to providing a means for better control of the factors that govern the combustion process in a diesel engine. However embodiments of the invention may be equally applied to combustion engines running on low grade and/or low octane rated fuels.